Apple trees require cross-pollination to set fruit; they are not self-pollinating. If there are other apple trees nearby they flower at approximately the same time, then the bees will likely cross-pollinate the trees. For a fuller description of the pollination of apple trees and the role of insect pollinators, see McGregor's "Insect Pollination Of Cultivated Crop Plants" http://www.ars.usda.gov/SP2UserFiles/Place/53420300/OnlinePollinationHandbook.pdf
